For semiconductor processes, a vertical heat-processing apparatus is known as a processing apparatus of the batch type for performing a heat process, such as oxidation, diffusion, annealing, or film-formation, on a number of semiconductor wafers simultaneously. In the vertical heat-processing apparatus, a number of wafers are arrayed and held at intervals in the vertical direction in a support member called a wafer boat. This support member is loaded into a vertical process chamber. The wafers are heat-processed while being heated by a heating mechanism disposed around the process chamber.
For example, in a film-formation process, a boat with target substrates held therein is accommodated in a reaction tube (process chamber), which is then set to have a predetermined pressure-reduced atmosphere. A predetermined process gas, such as a film-formation gas, is supplied into the reaction tube from below. Further, the reaction tube is heated to a predetermined process temperature by a cylindrical heater disposed therearound. As a consequence, a film is formed on the target substrates.
In general, a film formed on each target substrate by such a film-formation process tends to vary in film thickness between the central portion and peripheral portion. Specifically, for example, the film thickness on the central portion of the target substrate is apt to be smaller than that on the peripheral portion. Thus, it is difficult to process target substrates with high planar uniformity. It seems that the causes of this are as follows.
Specifically, in a vertical heat-processing apparatus as described above, target substrates are heated by a cylindrical heater disposed therearound. At this time, the peripheral portion of each target substrate relatively rapidly increases in temperature, while the central portion slowly increases in temperature. As a consequence, a temperature planar difference occurs on the target substrate. In addition, the target substrates held by the support member are supplied with a film-formation gas from the peripheral portion side of each target substrate. As a consequence, on each target substrate, the film-formation gas is higher in concentration at the peripheral portion, and lower in concentration at the central portion. In other words, a planar variation in film-formation gas concentration exists on the target substrate.
As described above, since the peripheral portion of each target substrate is subjected to a higher temperature and a higher concentration of film-formation gas, the film-formation reaction is more accelerated thereon, resulting in a larger film thickness than at the central portion of the target substrate. This makes it difficult to attain high planar uniformity. The planar uniformity of a characteristic of a target substrate, such as film thickness or film quality, tends to decrease also in, e.g., oxidation or diffusion.
In recent years, as semiconductor devices become smaller, target substrates require a process to be performed thereon with higher planar uniformity. For example, a ±1% or less film thickness tolerance is increasingly been demanded.
An example of current techniques for improving the planar uniformity of film thickness and film quality on a target substrate is a method of performing a heat process while rotating a support member (susceptor or boat) that holds a target substrate in a process chamber. In this case, the geometric center of the target substrate is aligned with the central axis of rotation of the support member, which ensures improvement in the planar uniformity. For this reason, when the target substrate is transferred onto the support member, the target substrate mount position on the support member is optimized. Ideally, a transfer device is controlled such that the center of thickness distribution of a formed film is aligned with the geometric center of the target substrate.
Specifically, for example, a heat process is actually performed on a target substrate, and the thickness of a formed film is measured to obtain information on the film thickness planar distribution on the target substrate. The film thickness distribution information obtained at this time takes the form of, e.g., almost concentric circles, and is highly symmetric relative to the film formation center of the film thickness distribution. Then, on the basis of the film thickness distribution information thus obtained, an operator obtains, by visual observation, a misalignment amount (eccentricity amount) of the film formation center (process center) of the film thickness distribution, relative to the geometric center of the target substrate. Then, the operator sets, by an empirical method, the control section of the transfer device to reduce the obtained misalignment amount. By doing so, a teaching operation is performed to control the operation of the transfer device, so as to optimize the target substrate mount position on the support member. The teaching operation needs to be repeated several times, such as three to five times.
However, the method described above takes a long time, such as about four to five hours, for the teaching operation, including time for measuring the thickness of a film formed on a target substrate. Accordingly, it is difficult to perform a predetermined heat process with high operating efficiency. In addition, positional change of the film formation center of film thickness distribution cannot be uniquely determined by positional correction of the target substrate mount position. Accordingly, it is very difficult to align the film formation center of film thickness distribution with the geometric center of a target substrate. As a consequence, it is difficult to perform a heat process on a target substrate reliably with high planar uniformity.